Security markers are used to authenticate items. For example, bank notes typically include security markers such as watermarks, security threads, holograms, kinegrams, and such like. Chemical and biochemical taggants are also used as security markers for items. However, in many cases such taggants must be removed from the item for authentication analysis. This is both time-consuming and expensive.
Optically based approaches, such as those using luminescent or, more commonly, simple fluorescent inks and dyes, are also used to authenticate items. Fluorescent inks and dyes emit light when excited by radiation of a particular wavelength. Information embedded in an item using fluorescent inks and dyes can be retrieved when the embedded mark is illuminated with radiation of an appropriate wavelength.
An example of a particular type of fluorescent ink is described in U.S. Pat. No. 5,256,193, which is hereby incorporated by reference. The following patents describe various security labeling and printing applications, and are hereby also incorporated by reference: JP 8208976; U.S. Pat. Nos. 4,736,425; 5,837,042; 3,473,027; 5,599,578; GB 2,258,659; 6,344,261; and 4,047,033.
Known fluorescent inks and dyes have the disadvantage that they have very broad emissions spectra, which limits the number of different dyes that can be used. For example, one ink may produce a color which spans from red through green in the visible spectrum. Another may produce a color which spans from green through violet. Thus, if these two inks are used in or on an item, it is difficult to use a third ink with them, because the first two inks cover the entire visible spectrum.
For many purposes it is, therefore, desirable to provide security markers having an emission spectrum comprising one or more narrow peaks. Similarly, it is desirable to provide security markers which are inexpensive to manufacture and incorporate in materials, difficult to counterfeit, and quick and easy to detect in situ.